Turbulence in shallow jet flows

Abstract

The general flow pattern of an open channel flow, downstream of a width restriction by two artificial dams, is analysed. A physical Froude-scaled model, under hydraulic rough conditions, with a significant large Reynolds number is used to ensure turbulent flow. Upstream of the dams the flow is uniform in transverse direction, in between and downstream of the narrow part a jet is formed. On both sides of the jet large eddies are formed bounded by the wall, the jet and the dams. Due to the large velocity gradient in transverse direction a mixing layer develops at both sides of the jet. The width of the mixing layer, as expected, grows with the downstream distance and exceeds the water depth. 2D structures are clearly visible by injecting dye. In the mixing layer besides the macro time and spatial scales, the small Taylor and Kolmogorov scales are present. Whereas the macro scales are well represented in the measured data, the small scales are impossible to mark due to limitations of the Doppler device. When there is initial no net momentum in transverse direction present the jet is expected to appear symmetrical. However the jet is aligned to one of the sides every time the model starts to run. The preference for one or the other side seems to be random and cannot be related to momentum in transverse direction in between the dams. During measurements the position of the jet is stationary. The fixed position of the jet during measurements can be related to the Coand? effect. When the flow is disturbed and transverse momentum is added to the upstream flow, the jet can be deflected. The position of the jet and the evolving mixing layers can be related very well to the measured velocities upstream. Due to the limitations of the used momentum balance equation and use of the mean velocity in the bottom friction calculation the measured head loss is large compared to the calculated dissipative terms (bottom friction and Carnot loss).